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Description/Abstract

The understanding of the effect of hydrate on marine host sediments is fundamental for the detection and quantification of gas hydrate from exploratory seismic survey data. For the interpretation of seismic data, effective medium models are used, which employ different theoretical assumptions to relate wave velocities to gas hydrate content of the sediment. Methane gas hydrates occur in situ in a variety of morphologies, generally classed as either pore-filling or grain-displacing. There are effective medium models for pore-filling morphologies, while for grain-displacing morphologies, only one such model exists in the literature. Thus the effect of morphology is poorly understood and this understanding is limited to pore-filling morphologies, despite the fact that grain-displacing hydrate inclusions will have a significant impact on the seismic signature of the sediment-hydrate system, and thus on the predicted quantity of hydrate. This paper presents a numerical modelling technique called computational homogenisation and applies it for the first time to gas hydrate-bearing sediments with grain-displacing morphologies. This technique has the ability to represent the geometry of hydrate inclusions explicitly and it considers the multi-scale nature of the material from a geotechnical engineering perspective. The effect of hydrate on the overall seismic properties of the host sediment is portrayed through simulations of nodular and simple vein morphologies with differing hydrate contents. Results show that morphology has a significant effect on the overall material properties, with the effect being more pronounced on the overall compressional wave velocity than on the overall shear wave velocity. The ratio of the two velocities (Vp/Vs) differs depending on the type of morphology and can be used to gain relevant insight by assisting in the differentiation between nodular and vein morphologies